The present invention relates to a heat exchanger for use in a refrigerator or an air conditioner in which a zeotropic refrigerant mixture is employed as an operating fluid and, more particularly, to a condenser or an evaporator of a heat exchanger of a cross-fin tube type, and a heat transfer tube suitably used for the heat exchanger.
A refrigerant HCFC-22 which has conventionally been used for air conditioners and the like is now regarded as a cause of environmental destruction. Especially, emission of this refrigerant into the atmospheric air has a serious influence on the ozone layer surrounding the earth. Therefore, researches have been made for various substitutes for the refrigerant HCFC-22.
As a result, it was found difficult to use a single refrigerant as a substitute. Consequently, application of a zeotropic refrigerant mixture consisting of two or three kinds of refrigerants has been studied.
However, the following problem is disclosed in "Heat Transfer Coefficient of HFC's Zeotropic Refrigerant Mixtures in a Horizontal Grooved Tube" (Collection of Lecture Theses at the 30th National Heat Transfer Sympostium of Japan, 1993, Yokohama, Vol. 1, PP. 337 -339, published on May 26, 1993). A plain tube and a tube with inner helical grooves (the so-called micro-fin tube) extending at one helical angle shown in FIG. 5 have conventionally been used for a single refrigerant in most cases. When the structure of such a conventional tube is applied to a heat transfer tube with a zeotropic medium in order to constitute an actual cycle system, the heat transfer performance which is a particular phenomenon to the heat transfer tube is unfavorably deteriorated. Accordingly, it is an important matter to improve the performance of a heat transfer tube in a heat exchanger by a novel structure.
More specifically, the conventional tube with inner helical grooves of a single kind exhibits an excellent heat transfer performance with respect to a single refrigerant. However, the HFC zeotropic refrigerant mixture consisting of two or three kinds of refrigerants, which has been regarded as the likeliest substitute for the refrigerant HCFC-22, can not produce an effect as high as the single refrigerant.
FIG. 9 shows test results of condensation heat transfer coefficient when the conventional tube with inner helical grooves is used, in which a curve a indicates a test result of the single refrigerant, and a curve b indicates a test result of the zeotropic refrigerant mixture. Obviously, the condensation heat transfer coefficient of the zeotropic refrigerant mixture is lower than that of the single refrigerant. The zeotropic refrigerant mixture used in the tests consists of HFC-32, HFC-125 and HFC-134a by the amount of 30 wt %, 10 wt % and 60 wt %, respectively.
It is a first object of the invention to provide a heat transfer tube for a zeotropic refrigerant mixture which exhibits a high heat transfer performance.
It is a second object of the invention to provide a heat exchanger or an air conditioner for a zeotropic refrigerant mixture which exhibits a high heat transfer performance.